Our paradigm of Mars has shifted several times, particularly during the past 40 years of spacecraft observations. Lowell's view of a world criss-crossed by canals built by a dying race of Martians was supplanted in the 1960s by Mariner views of a geologically dead world with an atmosphere too thin to support liquid water. Mariner 9's discovery of channels and volcanoes shifted the view to one of a planet where water existed in the past and led to renewed interest in the possibility that life might exist on our neighboring world. The Viking lander investigations quashed hopes of finding the martian soil teeming with microbial life, but refocused the interest in biological activity (past or present) to identifying localized oases. Recognition that we have samples of the planet's crust in the form of martian meteorites has greatly influenced our understanding of the planet's bulk chemistry and thermal evolution. The latest shift in our mental picture of Mars has revealed that water, either liquid or ice, has played a larger role throughout the planet's history than previously recognized. Our post-Viking view was that Mars had a thicker atmosphere and warmer, wetting surface conditions early in its history, but the post-Noachian period has been characterized by the cold, dry climate that we see today. Insights gained from MGS, Odyssey, MER, and MEx combined with increasing computational capabilities have revealed that short-lived climate excursions have occurred up through recent times, driven by changes in orbital parameters and the planet's obliquity.